Notify if WebGL is not supported

examples/layer-clipping-webgl.html

@@ -28,6 +28,10 @@
         </div>
       </div>
 
+      <div id="no-webgl" class="alert alert-error" style="display: none">
+        This example requires a browser that supports <a href="http://get.webgl.org/">WebGL</a>.
+      </div>
+
       <div class="row-fluid">
 
         <div class="span12">

examples/layer-clipping-webgl.js

@@ -4,6 +4,14 @@ goog.require('ol.View2D');
 goog.require('ol.layer.Tile');
 goog.require('ol.source.OSM');
 
+if (!ol.webgl.SUPPORTED) {
+  var info = document.getElementById('no-webgl');
+  /**
+   * display error message
+   */
+  info.style.display = '';
+} else {
+
   var osm = new ol.layer.Tile({
     source: new ol.source.OSM()
   });
@@ -91,3 +99,4 @@ osm.on('postcompose', function(event) {
     var gl = context.getGL();
     gl.disable(gl.STENCIL_TEST);
   });
+}